Method of treating bark



METHOD OF TREATING BARK Filed Jan-.7, 1946 A Q'DRY DARK HAMMER PHLL WlTH APPROX- l MESH SCREENED DISCHARGE CYLINDRICAL Es 5CREEH APPROX 4 MESH IMPURE CORK ROD PHLL HNFJ (YLINDRICAL FIBER AND 5C R EEN v AMORPHOUS APPROX. 40 MESH 7 MATERIAL PURIFIED CORK I: PROCESS sna /NA /E/\/ TUF? RAYMOND 5. HATCH 7 TTUF-QA/Exi Patented July 13, 1948 2,444,920 METHOD or TREATING max Raymond S. Hatch, Longvlew, Wash, auignor to i Weyerluuser Timber Company, Longvlew, 1 Wash, a corporation at Washington,

Application January '2, 1946. Serial No. eaacaa This invention relates to a method of treating bark forthe recovery of its content of cork. More particularly, the invention pertains to a method of separating from the barkof trees its cork con tent in a physical form which is relatively undamaged by the treatment to which the bark has been subjected, and which is substantially free from contamination.

It has been recognized in the prior art that the barks of many trees other than the cork oak contain substantial quantities of cork. The bark of the Douglas fir, for example, contains up to about 50% or more by weight of cork. The cork is present in the barks of such trees in the form of a8- gregates of cork cells. As compared with the other components of bark, for example, the sieve tubes, food storage cells, connecting cells, stone cells and fiber bundles, which components collectively comprise the bark phloem, the cork aggregates are relatively tough and resistant to abrasive or crushing action. Advantage has been taken of this property by prior art methods for separating cork from bark comprising subjecting the bark to a grinding or abrasive action calculated to efi'ect the differential pulverization of the bark constituents and subsequently separating, as by selective screening, the cork particles from the more finely divided content of the pulverized mass. The methods of the prior art have not, however, been designed to avoid damage to the cork particles. The conditions of treatment have been of such severity as to result in the tearing,

shredding and flattening of the cork cell aggre-' gates to a diversity of shapes and sizes with resultant loss of product during screening due to particular size reduction. Such treatment also has obvious adverse efi'ects on the properties of the cork and its suitability for application to subsequent uses. A further result of the distortion of the cork particles during the crushing step is that the screening operation is complicated and it is correspondingly dimcult to secure the complete separation of the cork from the other bark constituents. The cork fraction is therefore obtained in animpure condition contaminated by substantial amounts of other fibrous and amorphous materials.

It is, therefore, an object of the present invention to provide a method for the separation of cork from the other bark constituents.

It is another object to provide a method which accomplishes the recovery of cork from the bark of trees in high yields.

,Still a further object is to recover a fraction comprising cork by the treatment of the bark of 3 Claims. v(c1. 241-:14)

v 2 trees, which fraction comprises particles of cork which have been relatively undamaged by the treatment they have received.

Another object is to provide a method for the production of a cork product comprising particles of cork of substantially uniform size and shape.

Still a further object is to produce a cork product which is relatively clean and uncontaminated by the presence of other constituents of the bark from which said product is derived.

The operation of the hereinafter described process is schematically illustrated by the accompanying flow chart.

It has been discovered that the above and other objects of the invention may be achieved by subjecting pieces of bark to the action of means for breaking the bonds between the cork particles and the surrounding matrix by disintegration of the pieces into a mass in which the cork particles are to be found loosened but not entirely freed from particles of non-cork constituents of the bark. The pieces of bark are thus disintegrated without substantial reduction in the particle size of the cork and without tearing or shredding or crushing of the cork particles. The disintegrated mass is then subjected in a subsequent operation to a rolling, beating action having for its function the further pulverizing of the phloem and its relatively complete disengagement from the cork. This operation, therefore, completes the action started in the first operation, 1. e., the diflerential pulverization of the phloem, and, in addition, cleans the cork particles by beating them sufllciently to completely free them from anyadhering particles of foreign matter.

The first of the abovesteps, i. e., the disintegration of the pieces of bark, is preferably practiced upon bark which has been air dried and is in a state of subdivision such that it is conveniently introduced into and operated upon by the particular means employed for its disintegration. Such means must perforce be of sufficient strength and power and be so designed as to subject the bark to a uniform beating and crushing action in order to break down the bark structure and loosen the cork cell aggregates from the surrounding material. This may be accomplished in a swing type hammer mill which reduces the bark to a size suificiently small to pass through the openings of the screen with which the mill is provided. The screen openings may be varied according to the size or the cork aggregate desired and may vary from a minimum of /64 inch to 1 inch. 'Under certain conditions the cork aggregates are quite large, and it may be desirable to obtain from the bark a cork fraction having ag egates a largeas practically possible.

After the bark has been reduced -and disintegrated as in a hammer mill, it comprises a mixture of relatively large pieces, chiefly comprising the cork particles which have not been broken down by the treatment they have received, and finely divided material principally comprising fiber bundles and an amorphous material, which, being relatively friable in character, has been largely reduced in size and pulverized. The entire mass may, therefore, advantageously be screened for separation of the fines, comprising powdered phloem, from a coarser fraction which contains the desired cork. Any suitable screening device may be used for this step, as, for example, a rotary cylindrical screen. A screen size of between about and 50 mesh, preferably about mesh,

is preferably employed, since a screen of this size passes the powdered phloem but retains the larger particles comprising cork.

The coarse fraction resulting from the screensmall so that they may be separated from the cork. It also serves to clean the cork particles by completely freeing them from any material remaining adherent thereto. It is critical to the success of this operation and to the production of a cork product of the desired properties that the procedure employed be sufliciently severe to achieve the desired results but sufllciently gentle so that the cork particles themselves are not reduced in size or damaged by the treatment as by shredding, tearing or flattening. The desired selective action may be had, particularly where part of the original bark content has been removed as fines in a prior screening step as described above, by treatingthe mass in a rod or tube mill rotated at a conventional speed for a time sufficient to free the cork aggregates of the fine material adhering thereto.

It will be understood that the rate of treatment in a rod or tube mill depends on the amount of material which is being fed to such mill, and that the weight and size of the rods used for this purpose must be chosen with respect to the amount of material being treated, so that no severe crushing or tearing action will occur beyond that necessary to free the cork of the fiber and amorphous material. In a small experimental rod mill, the ratio and weight of the rods to the bark being treated would naturally be quite different from the ratio and weight of rods in a large mill. For experimental purposes this was done in a small rod mill operating on a. batch principle. In commercial operation the same end would undoubtedly be accomplished through the use of a rod or tube mill operating continuously. The coarse material would be fed into one end of this mill and the cork particles with the separated fines would be discharged from the other end of such mill into a suitable screening device for separating the fines.

The milled product is then screened for separation of the relatively coarse cork particles from the fines principally comprising fiber and amorphous material. Any suitable screening means may be employed to accomplish. this purpose.

. 4 v I about 30 and about mesh, preferably about 40 mesh, as described above.

ture is broken down or disintegrated by a beating Suitable means comprise, for example, a rotary cylindrical screen having a screen size of between action, as in a hammer mill. In the second, which is of less severity than the first, the disintegrated mass is treated in such a manner as to preferentially pulverize the non-cork portion of the bark while simultaneously disengaging the cork particles from any adhering material. Such a procedure is to be distinguished from one wherein the bark is broken down by severe treatment in a single step, with resultant damage to the cork, as well as from one in which the breaking down of the bark is effected in a series of operations of a similar character and comprising grinding or abrasive action which must of necessity abrade and comminute the cork as well as the other bark constituents.

The character of the treatment described herein is reflected in the properties of the cork product obtained therefrom. Said product comprises cork particles which are of a substantially uniform size and shape and are surprisingly clean and free from contaminating materials. The particles are obtained in the form of circular flakes or platelets having diameters of between about 3 millimeters and about 4 millimeters and thicknesses of between about 1 millimeter and about 2 millimeters. The surfaces of these particles are uniformly smooth and do not show evidences of tearing, shredding or abrasion. Although the yield of cork obtained is variable depending principally upon the species of tree which is used as a source of the bark processed, substantially all of the cork content of the bark, for example, between about 5% and about 50% by weight, based on the original weight of the bark, may be recovered in the above described condition.

In a typical instance cork was obtained from the bark of the Douglas fir by subjecting pieces of air dry bark having a moisture content of from 8% to 9% to the action of a swing type hammer mill operated at ,3600 R. P. M. The hammer mill was equipped with a screen having openings of approximately A inches, and the charge passed through the screen openings almost immediately.

The product resulting from the hammer mill treatment was then screened through a 40-mesh cylindrical screen. This resulted in the separation of the mass into a coarse fraction comprising 54.5% by weight of the total weight of the mass and a fine fraction comprising 45.5% by weight of said mass.

The coarse fraction was placed in a rod mill having rubber coveredxrods measuring approximately inch in diameter, The mill was operated at 50 R. P. M. for one-half hour. The mass was then screened, a IO-mesh cylindrical screen again being used. There was separated from the fines a 15% by weight (basis of the weight of the original bark) yield of cork particles in a physical form substantially as described above. The condition just described for the rod milling could obviously be changed over a wide range depending upon the ratio and weight of rods to material undergoing milling. The only precaution required is that the action of the rods should be such as to separate the fines without material reduction in size of the cork particles.

Having now described my invention and in what manner the same may be practiced, what I claim as new and desire to protect by Letters the size or the cork particles, screening on a screen'ot between about 30-and about 50 mesh for separation of the fines, tube milling the cork residue with a rolling, beating action to comany adhering material, and screening the resulting mixture for separation of the cork particles irom thefines.

2. A method of processing bark for recovery of its cork content which comprises disintegrating pieces of air dry bark in a hammer mill to eflect a cork content particle size in a range from "/u to 1 inch, screening the milled product on a screen of between about 30 and about 50 mesh for separation oi. the fines, rod milling the course residue for removing further pulverized non-cork constituents from the cork surfaces, and screening the product to separate the cork from the pulverized phloem.

3. A method of producing cork which comprises milling pieces 01 Douglas fir bark having a moisture content or between about 5% and about 20% by weight in a hammer mill for a time period sui'iicient to reduce the pieces to a cork content particle size in a range from "/u to 1 inch, thereby disintegrating the pieces of bark plete the pulverization of the content other than cork and to shake free the cork particles from without abrading or reducing in particle size the aggregates of cork cells contained therein,

screening the hammer milled product for separation of the fines on a screen of between about 80 and about 50 mesh, subjecting the coarse residue to a rolling beating action in a rod mill; to pulverize the non-cork content of the mass and shake free the cork aggregates from any adhering material, and screening the product of the rod mill in a screen having a mesh size of between about 30 and about for separation of the relatively large particles of cork from the more finely divided material comprising pre- 7 dominantly bark phloem.

' RAYMOND S. HATCH.

nnrnasncss crrnn The following references are of record in the file of this patent:

' UNITED STATES PATENTS Number Name Date 7 50,563 Dodge Oct. 24, 1865 1,317,852 Ashenhurst' Oct. 7, 1919 1,455,762 Howard May 15, 1923 1,598,328 Truax Aug. 31, 1926 1,607,828 Hoithofl Nov. 23, 1926 1,750,725 Miller Mar. 18, 1930 1,771,477 Alton July 29, 1930 1,784,762 Smith Dec. 9, 1930 1,807,383 Carnahan May 26, 1931 7 1,870,166 Bond Aug. 2, 1932 1,875,045 Leshner Aug. 30, 1932 1,880,045 Richter Sept. 27, 1932 1,996,177 Stephenson Apr. 2, 1935 2,362,528 Balz Nov. 14, 1944 2,364,721 Kassay Dec. 12, 1944 OTHER REFERENCES Chemical and Metallurgical Engineering, vol. 45, No.5, May 1938, pages 241 and 242. (Copy in 241-30.) q

Chemical Engineers Handbook, 1934 edition, M'cGraw-Hill Book 00., New York, page 1574. (CopyinDivisicn 25.) 

